5:15 PM - 6:45 PM
[S09P-25] Stress tensor inversion in the focal area of the 2016 Mw7.8 Kaikoura earthquake, New Zealand (2)
本研究では、ニュージーランドの南島北部で発生したカイコウラ地震を研究対象とし、地震のメカニズム解を複数決定することによって、本震前後の応力場を求めた。本震断層面上で余震が多く起きている場合、断層面が偏りを持って分布している可能性があるため、本震断層面上で発生した余震メカニズム解の影響を検討する必要がある。そのため、Hamling et al. (2017)で設定された20の断層モデルと余震メカニズム解とのKagan角(Kagan, 1995)を用いて、断層面上で起きていると考えられる余震を取り除いた後、応力テンソルインバージョンを行った。本震前の地震を用いた応力テンソルインバージョンの結果、Townend et al. (2012)で得られていた結果と同様の最大水平応力軸方向を得た。本震後の地震を用いた応力テンソルインバージョンの結果、南島北部のLondon hill断層付近の地域やKekerengu断層付近の地域では、Kagan角の閾値に依らず安定した横ずれ断層型となった。また水平最大圧縮軸方向はおよそN115Eであり、本震前について得られている方向と概ね変わらなかった。本震の震源付近ではより細かく分割して応力インバージョンを行った結果、南部の一部地域では逆断層型の応力場が推定され、その領域では本震時に逆断層の滑りが生じていたと推定される。今後は、本震の震源付近の地域や本震前の地震についてもメカニズム解を増やして安定した応力場を得られるようにし、更に細かい領域での応力場を推定する必要がある。
We study spatio-temporal changes of stress field surrounding the 2016 Kaikoura earthquake rupture in the northern South Island of New Zealand. Data from 51 temporary stations and 22 GEONET stations were used. Data collection was from March 2011 to December 2017. We derived focal mechanisms using HASH (Hardebeck and Shearer, 2002), By focal mechanism solutions of earthquake, we estimated the stress fields of main shock before and after Kaikoura earthquake. We used SATSI (Hardebeck and Michael, 2006) for stress tensor inversion. By using Kagan angle between main shock fault model and each focal mechanism, we remove aftershocks on the faults, and perform stress tensor inversion. We use the fault model of Hamling et al. (2017). The stress regime after the main shock in the areas near the London Hill Fault and near the Kekerengu fault are strike-slip fault type with SHmax orientation of ~N115E, which is almost same as for before the Kaikoura earthquake (e.g., Sibson et al., 2012; Townend et al., 2012), and stress tensors before and after the main shock didn't appear to change after the earthquake beyond the estimation error. In some areas of the south near the hypocenter of the main shock, the obtained stress tensor inversion is of this area. As a future study, we plan to obtain results of a stable stress tensor for before the main shock and in the area near the hypocenter of the main shock by increasing the number of focal mechanisms of earthquakes that we examine.
We study spatio-temporal changes of stress field surrounding the 2016 Kaikoura earthquake rupture in the northern South Island of New Zealand. Data from 51 temporary stations and 22 GEONET stations were used. Data collection was from March 2011 to December 2017. We derived focal mechanisms using HASH (Hardebeck and Shearer, 2002), By focal mechanism solutions of earthquake, we estimated the stress fields of main shock before and after Kaikoura earthquake. We used SATSI (Hardebeck and Michael, 2006) for stress tensor inversion. By using Kagan angle between main shock fault model and each focal mechanism, we remove aftershocks on the faults, and perform stress tensor inversion. We use the fault model of Hamling et al. (2017). The stress regime after the main shock in the areas near the London Hill Fault and near the Kekerengu fault are strike-slip fault type with SHmax orientation of ~N115E, which is almost same as for before the Kaikoura earthquake (e.g., Sibson et al., 2012; Townend et al., 2012), and stress tensors before and after the main shock didn't appear to change after the earthquake beyond the estimation error. In some areas of the south near the hypocenter of the main shock, the obtained stress tensor inversion is of this area. As a future study, we plan to obtain results of a stable stress tensor for before the main shock and in the area near the hypocenter of the main shock by increasing the number of focal mechanisms of earthquakes that we examine.